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FCC Auctions: More Potential For DTV Interference

The Notice of
Proposed Rule
Making issued
by the FCC on Sept.
28 provides a wealth
of information as how
the commission will go
about repacking broadcast
spectrum and auctioning
some as yet undetermined
portion of the UHF band.

From it, I learned that the present allocation
of Ch. 37 (608–614 MHz) to radio
astronomy might be changed to allow
broadcasters to use Ch. 37, while radio
astronomy would be given some other spectrum,
possibly near Ch. 37.

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This made me think about the double-conversion
tuners in some of the NTIA-approved
converter boxes. Some, if not all of
these have a first IF of 1222 MHz. This is the
second harmonic of the center frequency
of Ch. 37—611 MHz. This cannot be a
coincidence. Ch. 37 is the only TV channel
that has never been used for broadcasting.
Any strong signal on that channel could
generate second-order distortion products
(second harmonics) centered on 1222 MHz.

In my laboratory, I have some down-converters,
which I believed use a doublec onversion
tuner. So my colleague, Linley
Gumm, and I generated an ATSC Ch. 37
signal on my Rohde & Schwarz DTV signal
generator and fed this, along with a desired
signal, to these down-converters.

All four converter boxes were jammed
by a strong Ch. 37 signal. The interesting
thing is that this happens when the
unit is tuned to any TV channel from 2 to
51. It appears that the designers of double-conversion
tuners really knew what they
were doing in selecting this first IF frequency.
There really are few alternatives to
1222 MHz for double-conversion DTV tuners;
each has its good and its bad features.

So, what if the FCC allows broadcasters
to use Ch. 37? Trouble appears likely
for receiving devices with a first IF=1222
MHz, like all the double-conversion tuners
in my lab.

But, this is a hypothetical question. Unless
the FCC changes the rules, it simply
won’t be a problem. Or will it?

Another second-order distortion product
is the sum of the frequencies of two
signals: F1+F2. Suppose that Ch.
37–X and 37+X (X is an integer) are allocated
in the great repacking in the same
market. For example, let X=2. We would
have an undesired signal on Ch. 35
and another on Ch. 39. Their center
frequencies are 599 and 623 MHz. The sum
of these is 1222 MHz.

So we tried this experiment and found
that, compared to a single UHF signal on
Ch. 37, we had more interference
from this symmetrical pair of channels.
There were no double-conversion tuners
used in analog TV receivers, so this problem
was unknown until now.

There are many channel pairs that are
symmetrical around Ch. 37. How
many?

37 – 14 = 13 pairs. (Ch. 37 + Ch.
13 = Ch.50)

But would these far out pairs of UHF
channels cause interference?

So we did another experiment with
these four double-conversion tuners. We
used Channels 17 and 57 (X=20). That required
three frequency-agile ATSC signal
generators. I used mine and we borrowed
two more from Larry Sayer of Rohde &
Schwarz. Another was obtained from Bernard
Rate, senior engineer of GPA Associates,
a local consulting firm.

The interference was slightly less with
X=20 than X=2, but yes, any symmetrical
pair of signals can cause interference, not
just herringbone lines as was the case with
analog TV, but complete reception failure!

So it really doesn’t matter much whether
or not the FCC allows broadcasters to
use Ch. 37. These symmetrical pairs
of channels are the killers. And kill is what
they can do to any DTV signal to which the
viewer may have tuned in if it’s
weak enough.

In our experiments, we set
the UHF signal(s) to –17 dBm
per channel. We changed the
desired signal power until the
units under test locked to the
desired signal and produced
stable pictures with audio. Under
these conditions the minimum
usable ATSC signal was –46 dBm for
X=2, and –53 dBm for X=20—quite a ways
from its noise-limited value of –85 dBm.

We also found that Dmin varies linearily
with the U power. If we increased the power
per channel by 10 dB, Dmin followed.

Alas, what can be done about this newly
discovered interference mechanism?

The FCC could set up a program for owners
of NTIA-approved converter boxes that
fail after the spectrum repacking is accomplished.
The NTIA issued about 33 million
coupons with which people could buy an
NTIA-approved converter box for $40, a
real bargain. As a guess, there are probably
30 million converters in use. Of these, perhaps
15 percent have a double-conversion
tuner, so the number of replacement boxes
needed is approximately 5 million.

However, in many communities there
may be only a few of these symmetrical
channel pairs in operation. Also,
some of the converter boxes have
by now been replaced by new DTV
receivers. In any event, this problem
can be solved by the government
throwing money at it, and I think
that they must. After all, the NTIA approved
these converter boxes for its
$40 coupon program.

While I haven’t seen any of these
in stores for several years, a government
program would create a market
for possibly 4 million boxes.
Single-conversion converters from
the NTIA-approved list would reenter
the marketplace. The FCC
knows which converter boxes have
a double-conversion tuner. So, no
problem!

Charles Rhodes is a consultant in the
field of television broadcast technologies
and planning. He can be reached via
email at [email protected].